1. Field of the Invention
This application relates to shape memory devices, and more particularly to a spatially addressable shape memory device.
2. Description of Related Art
Materials which change their shape in response to external physical parameters are known and appreciated in many areas of technology. Shape memory alloys (hereafter "SMA") is a material that undergoes a micro-structural transformation from a martensitic phase at a low temperature to an austenitic phase at a high temperature. In the martensitic phase an SMA exhibits low stiffness and may be readily deformed up to 8% total strain in any direction without adversely affecting its memory properties. When heated to an activation temperature, the SMA becomes two to three times stiffer as it approaches its austenitic state. At the higher temperature, the SMA attempts to reorganize itself on the atomic level to accommodate a previously imprinted or "memorized" shape. When the SMA cools it returns to its soft martensitic state.
A shape may be trained into an SMA by heating it well beyond its activation temperature to its annealing temperature and holding it there for a period of time. For a TiNi SMA system, the annealing program consists of geometrically constraining the specimen, and heating it to approximately 520 degrees C. for fifteen minutes. Usually, functionally is enhanced by leaving in a certain amount of cold working by abbreviating the anneal cycle.
U.S. Pat. No. 4,543,090 (hereafter the "'090 Patent") discloses a catheter with two distinct SMA actuators. One actuator assumes a predetermined shape when heated to a predetermined temperature. The two actuators are coupled to each other with a coupling device so that when one of the actuator moves to its predetermined shape a force is applied to move the second actuator in the direction of the first actuator. Each actuator is only able to move to a single predetermined shape. The actuators do not include a heating device with at least two micro-fabricated address lines. The limitations of the '090 Patent are also found in U.S. Pat. No. 4,601,705.
It would be desirable to provide a shape memory alloy device that has a sheet of shape memory alloy where a section of the sheet can be selectably activated.